In a high-temperature solid-electrolyte fuel cell, the combustion reaction of a gaseous fuel with air is carried out electrochemically. The redox reaction takes place in the vicinity of the solid electrolyte which is provided with electrodes on both sides. Fuel molecules are oxidized by releasing electrons, while the oxygen is reduced on the other side of the solid electrolyte by absorbing electrons. The solid electrolyte separates the coreactants, prevents an electrical short circuit and provides for a substance equilibration since it has a high conductivity for ions accompanied at the same time by low conductivity for electrons.
Known high-temperature fuel cells have a solid electrolyte composed of fully stabilized yttrium-stabilized zirconium oxide (YSZ) which has an adequate ion conductivity at approximately 950.degree. C., at which conductivity the fuel cell can be operated with sufficiently low internal losses.
In order to obtain an operating voltage exceeding the potential difference of a single cell at the working point of the fuel cell, a plurality of fuel cells are connected in series. In the planar fuel-cell stack concept, this is done in a simple manner by stacking individual fuel cells one on top of the other, intervening bipolar layers and plates composed of a so-called ICM material (interconnection material) ensuring the sealing of the gas spaces and the electrical connection between two adjacent individual cells.
In implementing the planar fuel-cell concept, a few important problems still occur which have hitherto made the economical use of high-temperature fuel cells difficult. Thus, there is a problem, for example, in the selection of the materials which are used for the fuel cells and which have to withstand the high operating temperatures of up to approximately 1100.degree. C. accompanied at the same time by high pressures of the fuel gas or of the oxygen needed for the combustion of up to 16 bar. As a result of the high operating temperatures, chemical and physical processes which endanger the long-term stability of the high-temperature fuel cell set in, particularly at the interfaces between two adjacent layers of different composition. As a result of interdiffusion between individual layers, the chemical composition of the layer materials alters and, consequently, so also do their properties. As a result of sintering and recrystallization processes, particularly in the cermet (=ceramic metal) used as anode material, both the electrochemical activity and the stability or service life of the high-temperature fuel cell decrease.
A further problem is to join both individual fuel cells and entire fuel-cell stacks together in a manner which is gastight with respect to the high operating temperatures and the high pressure.